1. Field of the Invention
The present invention relates to a radio receiver, and more particularly, to a novel and improved radio receiver superior in anti-interference performance and sensitivity.
2. Description of the Prior Art
As well known, the performance characteristics required of radio receivers are generally the following four ones:
(1) Sensitivity PA1 (2) Anti-interference performance PA1 (3) Fidelity PA1 (4) Stability PA1 (1) Their central frequency is variable; PA1 (2) They each have several filters designed for different frequencies, one such filter being used at any given time; or PA1 (3) They are wide-band filters which let pass all the signals of frequencies within the variable band.
Radio receivers which meet all the above four requirements can of course be accepted as quality apparatus. Along with the ever-increasing number of broadcasting stations, however, the required performance (2) above, say, the anti-interference performance, has recently been more and more important in this field.
A general means of improving the anti-interference performance is to use narrow-band filters; FIG. 1 shows the construction of a conventional superheterodyne receiver equipped with such filters. In this Figure, the reference numeral 1 denotes an antenna, 2, 4 and 7 indicate filters, 3 refers to a high frequency (HF) amplifier, 5 to a frequency converter, 6 to a local oscillator, 8 to an intermediate frequency (IF) amplifier, 9 to a demodulator, 10 to a low frequency (LF) amplifier and the numeral 11 refers to a speaker. The anti-interference performance of the radio receiver of a such construction will be discussed below.
The antenna 1 does not meet our expectation in respect of the frequency selectivity. Suppose that an HF signal received by this antenna 1 is amplified as it is by the HF amplifier 3; harmful phenomena such as intermodulation or cross-modulation will be caused by the nonlinear effect of the amplifier 3. To avoid this, the filter 2 is connected down stream of the antenna 1.
The frequency converter 5 is substantially a nonlinear circuit which will incur an image frequency interference in addition to the intermodulation or cross-modulation, and to avoid the above, the insufficient frequency selectivity of the filter 2 is covered by another filter 4. The numeral 6 denotes a local oscillator. The filter 7 is provided to select a signal which has been converted in frequency to in intermediate frequency.
As described in the above, the conventional radio receivers are usually provided with 3 filters. Of them, the filters 2 and 4 have been as follows, in case of the received frequency being variable:
However, the filters in (3) above are not substantially narrow-band ones, and consequently are poor in performance as to anti-interference. Although the filters in (2) above may be used to some extent for a narrow band, many such filters are necessary so that the costs and mounting space will be great. Further, with the filters in (1) above, it is difficult to provide a sufficient frequency selectivity to suppress signals other than a desired signal.
To obtain a sufficient frequency selectivity using the filter in (1), it is necessary to provide an increased number of filter stages, which, however, will lead to an increase of loss. More particularly, if a such filter is used as the above-mentioned filter 2, the noise factor of the radio receiver is adversely affected, and a tracking error of the filter takes place; thus, it is not possible to increase the number of filter stages at random.
To suppress the interference from adjoining channels, there is provided an IF signal selecting filter 7 downstream of the frequency converter 5. This filter 7 may be such that its central frequency is fixed and its subject frequency is low. Accordingly, an inexpensive filter of good selectivity can possibly be used.
It is apparent that the frequency selectivity of the filters 2 and 4 are insufficient to suppress the intermodulation or cross-modulation caused by the HF amplifier 3 and frequency converter 5. The basis of intermodulation occurrence will be described herebelow.
Assume that the received frequency is fd and that there are interference waves fd.+-..DELTA.f and fd.+-.2.DELTA.f adjacent to the frequency fd. There occurs in the HF amplifier the frequency fd from the tertiary nonlinear term as below: EQU 2(fd.+-..DELTA.f)-(fd.+-.2.DELTA.f)=fd
This means the occurrence of interference from the waves adjacent to the frequency fd. This frequency varies depending upon the kind of communication being made, and is on the order of 10 kHz in some cases.
Accordingly, to prevent any interference, an HF filter is necessary which can sufficiently suppress the frequency components very near the frequency used in communication. However, there have been available so far only piezoelectric HF filters using crystal, of which the central frequency is fixed. Because of this fixed central frequency, such filters are those in (2) above.